Battery safety and solar

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question

campsite asked Jan 17, '24

Battery safety and solar

Hi guys,

I finally finished my campervan build and found my two new batteries to be failing.

My conclusion is that the batteryprotect never activates because the minimum voltage is not met during daytime.
I have a Multiplus 12/2000/80, a MPPT 150/60 (with 2x200w PERC) and 2x 105Ah Calcium traction batteries.

We have no sun most of the time this time of year, but the MPPT is giving between 13 and 14 volts, so starts charging with allmost no current. I draw more current than it gets, so the batteries are draining without the BMV seeing the voltage drop below 11.9v.

What can i do about this and protect my (second) new batteries?

battery charging Battery Protect battery status

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regman54 commented · Jan 26 at 02:45 PM

"What can i do about this and protect my (second) new batteries?"

How about a backup plan? Thanking outside the box? Solar is great while the sun shines but as you found out not reliable.

What I do in your situation, where I need to protect the house batteries from discharging too far because of a lack of solar, is either plug into shore power when available or use a small 2000W gas powered inverter generator to recharge. They are small, light weight, and work in any weather condition.

Just a thought. Cheers.

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kevgermany ♦♦ regman54 commented · Jan 27 at 08:02 PM

@Campsite

To add. You need to think of this as two issues. One is inadequate solar, which @regman54 has addressed in supplemental charging. The other is system.

There's no protection in your system to prevent excessive discharge. It relies on you checking the BMV and taking action. If there was protection, you would find everything shutting down. Probably when you need it most. You can set the cut out voltage higher in the multiplus, but this may trip you up with heavy loads and full/nearly full batteries. And it won't stop DC loads.

Turning off the inverter when you don't need AC will help. Minimising DC loads will help. So will more PV. It's surprising how multiple small loads add up. Even with LED lights.

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campsite kevgermany ♦♦ commented · Jan 27 at 10:00 PM

Thanks for the replies.

@regman54 , as for backup i got some torches and a bbq, but that’s not the point here.

The question is not to protect de batteries because of lack of solar, it is to protect the batteries in general, but the bmv is failing to do so because of solar.

@kevgermany , so the BatteryProtect is useless as it only cuts off on a voltage it does not reach?

What would i need to check on the bmv? I found the percentage is not correct, as is the voltage, so i can only check the total drawn Ah’s?

I set the multiplus to 12v, but it also does not cut off on time as the voltage is still around 13v because of the mppt.

saving loads is not the point here.

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kevgermany ♦♦ campsite commented · Jan 28 at 05:59 AM

BMV SOC will drift unless batteries reach full charge. Over time this can be large. Due to the way batteries work.

If the BMV battery settings are correct it will resync to 100% automatically every time it's fully charged.

Voltage is only useful as an indicator of SOC when batteries are resting and have been resting for a few hours.

Also even though you have lead/calcium, long periods of partial charge will cause damage.

Bottom line is you must charge fully regularly.

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regman54 campsite commented · Jan 29 at 04:47 PM

@Campsite "as for backup i got some torches and a bbq, but that’s not the point here."

Just to be clear, my suggestion to use a gas powered generator as a supplement is for recharging the batteries fully IF solar input is insufficient. I did not mean to use it as an alternate power source for torches or a BBQ, etc. although it would certainly suffice for that purpose if needed.

Kev is correct when he says voltage is only useful as an indicator. A voltmeter will not show an accurate state of charge (SOC) after both charging and discharging. The battery needs to rest for several hours or even overnight to get an accurate SOC. And yes, batteries need to be fully recharged frequently to remain healthy.

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campsite regman54 commented · Jan 29 at 06:07 PM

Thanks for the time and effort.

Understood. I’ll make sure i find ways to recharge. Options are solar, grid or with the engine running. During offgrid stays i won’t be using AC or warm water, so that will help a lot.

Still trying to figure out why i can only get around 40Ah out of my new 210Ah pack. If i can get 50% it would be enough for a few days offgrid without solar nor driving.

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matt1309 commented · Jan 17 at 03:57 PM

Hi @Campsite

If i've understood correctly and if you've wired it correctly that's not how the system will behave.

ie say you've wired battery -> BMV -> loads (either mutliplus or other DC loads).

similarly MPPT is after the BMV so battery -> BMV -> bus bar <- mppt.

What you're saying is the MPPT is lifting the voltage of the system by charging (but only just charging) and this stops BMV noticing voltage dropping below minimum?

However this is not what would happen. For example say mppt is charging with 50w, the charging elevates the system voltage by say 1volt. When you now draw 100w, the voltage of the system will drop that 1v it's elevated and then some to get the power out.

What you're describing where the BMV doesnt detect the battery dropping below minimum because MPPT is holding up the voltage isnt happening.

Only scenario i can see where you're battery is dropping below minimum is if you have loads attached to battery before BMV (possible). Or loads are staying on even if BMV is flagging minimum (check multiplus settings to make sure this isnt happening).

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Alexandra ♦ matt1309 commented · Jan 17 at 05:37 PM

14v from solar is so low it is not even worth having those two solar panels on the mppt. The start voltage is 5v above battery.

Put simply they need to be in series, not parrallel for now. But get more solar.

How is everything programmed?

Possibly a picture/diagram of the set up.

Calcium reactions want full charge everyday. So that is the main priority. 11.9 is too low anyway. Set your cut off by 12v.

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campsite matt1309 commented · Jan 26 at 01:23 PM

Hi, i tested your theory and have to say i don’t see the same.

Battery is at 12.22, with a draw of 0.8a, mppt starts charging and puts in 1a at around 13.24 (mppt says 1.8a, the extra 0.8 i cannot see as it does not go through the shunt)

Now putting a load on it of 5a, it still is around 13.20.

@alexandra, the pv outputs around 40v at time of this test

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Alexandra ♦ campsite commented · Jan 30 at 05:35 PM

By more solar, I mean the mppt starts up earlier and powers down later. So a longer solar day. So it is important to have higher PV voltage in strings. (Glad to see they are at 40v got a different impression from your original post.)

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Modifications Space

Node Red Space

Answer 1 · 2024-01-28T03:00:50Z

Paul B answered · Jan 28 at 03:00 AM

how about a wiring diagram for how you have set everything up showing all components. pictures would also be helpfull

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kevgermany ♦♦ commented · Jan 28 at 06:00 AM

@Campsite

And a full list of equipment.

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Answer 2 · 2024-01-29T14:55:57Z

campsite answered · Jan 29 at 02:55 PM

@Paul B

This is somewhat how it sits right now. I got some new batteries and the BatteryProtect is out of the game at the moment.

@kevgermany Thanks for your advice, will make sure they are fully charged most of the time as much as i can.

Wonder happened and i now see voltage and voltagedrops correctly in the BMV and it also alarmed me about low voltage during solarcharging with more draw than input! (although the mppt and bmv currently see 12.02 and the multiplus 12.34)

Only thing left is why i have 12v after 30Ah consumed, shouldn't i be able to consume around 100Ah? I fully charged overnight and the BMV stated 100%.
I turned on a vacuum for a few minutes sitting stable at -80Ah draw to consume 30Ah an hour ago, it now says 82% at 12.02v.

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Answer 3 · 2024-01-29T15:41:50Z

kevgermany answered · Jan 29 at 03:41 PM

Would be good to see the BMV battery settings.

The diagram looks good. Assume the batteries are in parallel, hard to tell on the phone.

BMV calculated SOC from the current it measures, subtracted from 100%. Will reset to 100% when charged.

Battery voltage drops under load. Recovers partly over time. But 12V@80% seems a touch low.

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pwfarnell commented · Jan 29 at 05:05 PM

This could be one of those cases with low solar and default BMV settings that cause the SOC to reset to 100% too early, hence the under charging. See what @Campsite comes back with.

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campsite pwfarnell commented · Jan 29 at 06:12 PM

@pwfarnell Yeah seen vids about that, not the case in this example, but managed to solve it miraculously.

@kevgermany thanks for your time so far. Here are my settings

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pwfarnell campsite commented · Jan 29 at 07:20 PM

The SOC will reset to 100% if you have above 13.6V and a current of less than 3A, which in winter solar charging can occur in low light while the batteries are much less than 100% full. If you increase the charged voltage on the BMV to 14.2V then the reset will only occur when the batteries are truly full. You may find that you are not getting to 100% in the winter depending on how much you discharge the batteries.

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kevgermany ♦♦ pwfarnell commented · Jan 29 at 07:37 PM

@Campsite

You've increased charged detection time, this may not be enough. Agree on the charged voltage with @pwfarnell , with the caveat that it should be just below the absorption setting in the MPPT.

Your 100% reading is false as a result of the settings. These would work for grid charging.

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campsite pwfarnell commented · Jan 29 at 07:57 PM

Thanks both, will change right away.

Although i am still wondering why i am missing 50Ah to use, even though it’s winter

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kevgermany ♦♦ campsite commented · Jan 29 at 10:05 PM

As said, the 100% is being set wrongly. So the missing Ah are never there due to undercharging.

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campsite kevgermany ♦♦ commented · Jan 30 at 07:21 AM

Does the multiplus stop charging because of the bmv’s SoC? I found it has his own SoC also that looks almost the same as the bmv

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pwfarnell campsite commented · Jan 30 at 08:31 AM

If the battery monitor in the Multiplus is enabled and as you appear to have Venus OS running (Cerbo GX or equivalent) then the Multiplus will use the SOC from the Cerbo. You can then program the Multiplus to shutdown at a specific SOC, but it is not set to do this by default, low voltage shutdown is set by default. These shutdowns are programmed directly on the Multiplus not via the Cerbo.

As you have Venus OS running, consider turning on DVCC under settings and SVS and SCS, share voltage sense and share current sense then all your equipment will use the BMV voltage and current readings. Once these are enabled you can go to the MPPT and set absorption the absorption time to fixed at a long duration, say 8 hours and to end on tail current of say 2.0 or 2.5A so that the MPPT does not go onto float until the batteries are completely full and this tail current is lower then the reset tail current on the BMV. You just need to check that the tail current does fall below this value when you know the batteries are well charged.

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campsite pwfarnell commented · Jan 30 at 04:10 PM

So the Multiplus won't stop charging because of my BMV?
Then the batteries should be full, as i remove the gridpower after the Multiplus went into storage, so i'm still missing 50Ah to consume.

I got VenusOS running on a raspberry with usb diy cables that tent to lose connection. I'll buy some Victron cables and look into DVCC! thanks again for all the info!

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kevgermany ♦♦ campsite commented · Jan 30 at 04:54 PM

No, it works on voltage. BMV SOC is too unreliable.

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campsite campsite commented · Jan 30 at 07:15 AM

Multiplus is allmost in storage mode, lets hope i get more Ah this time. Battery supplier says to ‘wake’ the new batteries a bit by charging and discharging a few times.

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Answer 4 · 2024-01-30T01:01:58Z

Paul B answered · Jan 30 at 01:01 AM

Please Note that the batteries that you indicate you are using the VDC31m These are Car starter batteries and from what I have read on the web these are NOT deep cycle batteries - thus if you are cycling them they will fail very very quickly.

In preference when it becomes time to replace them a 1 x 100 Amp hr Lithium phosphate battery would be a far better choice,

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Answer 5 · 2024-01-30T16:17:05Z

campsite answered · Jan 30 at 04:17 PM

@Paul B I already bought these after a long search. The lithiums are too expensive here and i planned to use 100Ah of the 210Ah pack. When these batteries are done i'll go Lithium! (hopefully after a few years)

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Answer 6 · 2024-01-30T22:15:35Z

Paul B answered · Jan 30 at 10:15 PM

Just be prepared for a very quick failure life under 1 year. 50% depth of discharge will kill them very quickly., try DOD at around 10 to 20% and you may get some life out of them but not much. Anyway good luck with them and I hope you get a good run with the batteries.

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Answer 7 · 2024-02-22T11:44:22Z

campsite answered · Feb 22 at 11:44 AM

Hi everyone,
update: i got fresh new batteries from the supplier and everything is working like a charm,
except the SOC percentage. It correctly resets to 100% at the correct moment, but 97% is sometimes 10Ah consumed and sometimes near 20Ah consumed. Still am clueless what else to set to make this better..

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pwfarnell commented · Feb 22 at 12:34 PM

This is how lead acid batteries work and the BMV trying to account for this. If you look at the specs for a 100Ah battery it will be 100Ah if discharged at 5A (C20 100/5=20). However, if it is discharged at say 50A it will only be able to provide say 60Ah. So if you discharge 20Ah slowly it will show an SOC of 80%. If you discharge it fast it will sow an SOC of 66.6%. This is called the Peukert effect and the BMV tries to allow for this, hence SOC and Ah consumed can be slightly different.

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campsite pwfarnell commented · Feb 24 at 06:21 AM

Aha thanks, that makes sense! Could you explain how to calculate the peukert effect?

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pwfarnell campsite commented · Feb 24 at 09:14 AM

There is a section in the BMV manual on the Peukert effect. This references a Victron calculator that you can use if you have a datasheet for your batteries with the Ah ratings at different discharge currents.

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